Soil pH is a measurement of the alkalinity or acidity of soil. pH is measured on a scale of 1-14, with 7 as the neutral mark, anything below 7 considered acidic soil and anything above 7 considered alkaline soil.
Technically, pH is a gauge of the hydrogen-ion concentration in the soil. For the gardener's needs, it is enough to know whether your soil is alkaline or acid because certain nutrients can only be accessed by plants when the soil pH falls into an acceptable range.
Most plants prefer a somewhat neutral pH, anything from 6.2 to 7.0. However there are many plants that are more specific in their pH needs, such as blueberries which like a very acidic soil and lilacs that prefer a more alkaline soil.
Acid soils are often referred to as "sour" and alkaline soils "sweet".
Examples: Don't just throw lime on your lawn every year out of habit. Test your soil's pH to see if is it acidic and if lime is even needed.